Twitch-dl

* Download VODs to cache dir instead of temp dir (105, 115, 164)
* Add `download --cache-dir` to allow user to override the default cache dir
* Add `cache` command for viewing cache usage and clearing cache
* Chat rendering improvements: align fonts to a common baseline, scale badges
and emotes

* Fix breaking bug in chat renderer

* Embed additional metadata to videos: `date` (publish date) and `show` (game
name)

* When downloading a video with `--start` and/or `--end` offsets, crop the video
to the exact times given, used to be rounded to the nearest VOD boundary
(usually 10s)

* Add new options when prompting if a file should be overwritten: overwrite,
overwrite all, skip, skip all, abort (thanks NuVanDibe)
* Embed chapter metadata in video, allows skipping to chapter if video player
supports it
* Increase default worker count to 10, seems to improve speeds
* Fix a bug where some clip slugs would not be recognized
* Add `--skip-existing` option to `download` command
* Add `--json` option to `chat` command
* Improvements to chat renderer

* Add `chat` command for rendering twitch chat as a video which can then be
embedded into a downloaded stream using ffmpeg.